For a given airspeed, an aircraft may consume less fuel at a higher altitude than it does at a lower altitude. In other words, an aircraft may be more efficient in flight at higher altitudes as compared to lower altitudes. Moreover, bad weather and turbulence can sometimes be avoided by flying above such weather or turbulence. Thus, because of these and other potential advantages, many aircraft are designed to fly at relatively high altitudes.
As the altitude of an aircraft increases, from its take-off altitude to its “top of climb” or “cruise” altitude, the ambient atmospheric pressure outside of the aircraft decreases. Thus, unless otherwise controlled, air could leak out of the aircraft cabin causing it to decompress to an undesirably low pressure at high altitudes. If the pressure in the aircraft cabin is too low, the aircraft passengers may suffer hypoxia, which is a deficiency of oxygen concentration in human tissue. The response to hypoxia may vary from person to person, but its effects generally include drowsiness, mental fatigue, headache, nausea, euphoria, and diminished mental capacity.
Aircraft cabin pressure is often referred to in terms of “cabin altitude,” which refers to the normal atmospheric pressure existing at a certain altitude. Studies have shown that the symptoms of hypoxia may become noticeable when the cabin altitude is above the equivalent of the atmospheric pressure one would experience at 8,000 feet. Thus, many aircraft are equipped with a cabin pressure control system to, among other things, maintain the cabin pressure altitude to within a relatively comfortable range (e.g., at or below approximately 8,000 feet) and allow gradual changes in the cabin altitude to minimize passenger discomfort.
In addition to a cabin pressure control system, many aircraft also include an environmental control system (ECS) that supplies temperature-controlled ECS air to the aircraft cabin, and which also improves passenger comfort. Typically, a flow of bleed air from one or more of the aircraft engines is supplied to the ECS, which in turn conditions the bleed air and supplies the ECS air to the aircraft cabin. The ECS air, when flowing into the aircraft cabin, will also pressurize the aircraft cabin and cause a change in cabin altitude. Thus, the cabin pressure control systems in such aircraft typically include at least an outflow valve and a controller. The outflow valve is mounted on the aircraft bulkhead and, when open, fluidly communicates the aircraft cabin to the environment outside of the aircraft. The controller implements various control laws and supplies appropriate valve control signals to the outflow valve that modulates the position of the outflow valve. As a result, the ECS air supplied to the aircraft cabin is controllably released from the aircraft cabin to the environment outside of the aircraft to thereby control aircraft cabin altitude.
When an aircraft that is pressurized as described above descends, the aircraft engine throttles are typically retarded, resulting in decreased bleed air pressure. However, as may be appreciated, additional bleed air flow may be needed to increase cabin pressure (decrease cabin altitude) during aircraft descent. In some aircraft, if additional bleed air flow is needed during descent, the additional bleed air is supplied from another portion of the aircraft engine (e.g., high pressure compressor) by opening a valve. Although this methodology generally works well, it does suffer certain drawbacks. Specifically, during some aircraft descents the aircraft may experience bleed air pressure fluctuations and concomitant cabin pressure fluctuations. Both of these fluctuations can be disconcerting to both the flight crew and passengers.
Hence, there is a need for a system and method of controlling aircraft cabin pressure, and more specifically bleed air supply pressure, during aircraft descent that does not result in disconcerting bleed air pressure fluctuations and/or cabin pressure fluctuations. The present invention addresses at least this need.